Variance in reef spatial structure masks density dependence in Coral-Reef fish populations on natural versus artificial reefs

Understanding the role of density dependence in the regulation of marine fish populations is particularly crucial as species face increased risk of extinction. Yet, spatial and temporal heterogeneity can easily mask density dependence. By experimentally manipulating group sizes of the yellowhead wrasse Halichoeres garnoti on both natural and artificial reefs, I determined that the relationship between density and mortality varied with reef spatial structure. On natural reefs, mortality rates were highly variable among reefs. However, losses also resulted from emigration to neighboring patches, and thus persistence of fish remaining on natural reefs was approximately density-dependent. On artificial reefs, mortality was density-dependent on reefs that were spatially isolated, and high and density-independent on reefs that were aggregated. Emigration was virtually zero among these reefs. Overall predator visitation rates were significantly higher to artificial than natural reefs, but a greater diversity and size range of predators were present on natural reefs. Based on observations that yellowhead wrasse formed tighter aggregations in the presence of predators (rather than seeking shelter in the reef), differences between artificial and natural reefs were likely not related to differences in reef complexity. Instead, on artificial reefs, standardizing reef spacing resulted in density-dependent versus density-independent mortality according to reef isolation. In contrast, heterogeneity in reef spacing among natural reefs likely caused variation in predation risk that in turn caused high variability in mortality rates. Small-scale spatial variation in predation may play an important role in the population dynamics of species that occur within patchy environments.